1. Field of the Invention
The present invention relates generally to visible light communication (VLC), and more particularly to multi-input multi-output (MIMO) orthogonal frequency division multiplexing (OFDM) VLC.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 And 37 CFR 1.98.
Visible light communication (VLC) is a short range optical wireless data transmission method that uses the illumination infrastructure as wireless access points. VLC systems are based on the principle of modulating light emitting diodes (LEDs) at very high speeds without any adverse effects on the human eye and illumination levels. LEDs are increasingly used both indoors (e.g., home and office lighting, etc.) and outdoors (e.g., street lights, traffic lights, vehicle front/rear lights, etc.). VLC uses 380-790 THz frequency bands and advantageously provides a signal not easily disturbed between a transmitter and a receiver.
The dual use of LEDs for both lighting and communication purposes is a revolutionary solution and has the potential to open a new era in wireless communications. The design of ultra-high-speed VLC systems (on the order of multiple gigabits per second) requires the development of advanced and custom-designed physical layer technologies taking into account the inherent characteristics of optical signal propagation.
Initial research and development in VLC has included non-adaptive systems which use fixed transmission parameters. Such systems fail to provide a satisfactory performance in time-varying channel conditions.
Adaptive radio-frequency (RF) wireless systems have been previously used in practice. However, such works have been designed for RF frequencies and cannot be applied to VLC systems with different sets of requirements and inherently different characteristics. For example, unlike RF signals, optical signals must be real and non-negative. Furthermore, VLC systems need to be designed taking into account their dual use; i.e., the functional use for illumination as well as communication.
Adaptive transmission in the context of VLC has been explored. Some of this research assumes single-carrier transmission under the assumption of frequency-flat channels. This is not realistic for the targeted ultra-high speeds on the order of gigabits per second.
Multipath VLC channels will exhibit frequency-selectivity at high data speeds and result in severe intersymbol interference (ISI). To handle this, adaptive VLC systems are built upon multi-carrier architecture and deploy orthogonal frequency division multiplexing (OFDM). In this technique, the high-rate data stream is de-multiplexed and transmitted over a number of frequency subcarriers. If appended cyclic prefix duration is larger than the delay spread, ISI can be neglected simplifying the receiver complexity. Different from its RF counterparts, the implementation of optical OFDM (O-OFDM) requires certain modifications to ensure the non-negativity of the intensity-modulated optical signal.
Previous research considers the combination of multi-input multi-output (MIMO) and OFDM, but this research has often been for non-adaptive systems or for a fixed MIMO type (i.e., spatial modulation, and the MIMO type is not changed in an optimal manner).
As a result of the above-mentioned drawbacks and the insufficiencies of previous solutions in VLC, improvements are required to be made in the related technical field.